It is well known that electronic logic circuits, such as, AND gates and the like, are used extensively in computer, communication, control and signaling apparatus. In the present case, the solid-state fail-safe AND circuit is preferably utilized in a vital-type of logic network and may form part of a speed command decoder of an automatic vehicle speed control system. In automatic vehicle speed control systems for railroad and mass and/or rapid transit operations, it is of paramount importance and, in some cases, an authoritative requirement, to ensure that certain critical portions or sections of the system must operate in a fail-safe manner. That is, during an unsafe circuit malfunction or critical component failure, the system must not be capable of exhibiting a less resistive condition. In practice, fail-safe operation may be considered to mean that any conceivable failure will result in a condition at least as restrictive or as safe as that preceding the failure. For example, no conceivable failure in a vehicular speed control system should be capable of issuing a command signal for a higher or increased speed. In most railroad and/or rapid transit speed control systems, it is desirable that a failure should result in a more or most restrictive condition, namely, the stopping of the train or vehicles.
While numerous static AND logic circuit arrangements are well known in the art, only a limited number of AND gating circuits are capable of meeting the above-noted criteria. As a rule, conventional or nonvital semiconductor logic circuits cannot be used in a vital speed control system because of certain shortcomings which result in unsafe operation. For example, the previous ON-OFF or saturation cut-off gate circuits generally produced a zero voltage (OV) or a positive or negative value (+V) or (-V) dependent upon the conductive condition of the solid-state or active elements. It will be appreciated that in such ON-OFF gates, a fully conductive transistor generally cannot be distinguished from a short circuited condition since in each case a saturated current condition exists. In addition, a nonconducting transistor cannot normally be differentiated from an open-circuited element since both resemble a cut-off condition. Obviously, such solid-state AND gates are unacceptable and cannot be tolerated in systems and apparatus which must operate in a fail-safe manner. While a number of attempts have been made to ensure fail-safe operation of these previous nonvital logic circuits, such endeavors usually involved the addition of auxiliary components and elements which materially increased the complexity and intricacy of the basic gating network. Such supplementary devices not only increased overall cost but also decreased the reliability of the logic circuit since any additional element is itself susceptible to failure and malfunction. There are other solid-state gating circuits in existance which utilize a.c. and d.c. input signals for performing an AND function in a vital fashion. However, it is desirable to provide a fail-safe electronic AND gate which is responsive to the presence of a plurality of d.c. inputs to safely produce a modulated output.